viernes, 7 de junio de 2013

European history is sprinkled with references to episodes when rain or snow turned brown, yellow, and even dark red. So-called “blood rains”
are mentioned by Cicero, Geoffrey of Monmouth, and writers throughout
the Middle Ages. The events were often taken as dreadful omens that
human suffering was imminent. When red rain drenched the Roman Senate in
191 B.C., Roman historian Livy described the response: “Being disturbed
by these prodigies, the Fathers decreed that the consuls should
sacrifice full-grown victims to whatever gods it seemed proper.”Thousands of years later, red rains still occur periodically in
Europe, though with less dramatic effect. They generally receive no more
than a brief mention by a news outlet
or a meteorologist because we now understand what causes them: strong
storms picking up Saharan dust and lofting it toward Europe. Colored
rain (or snow) occurs when dust plumes and storm systems meet, though
only when raindrops fall through a dust plume beneath the level of the
clouds.Dust storms are common throughout the Sahara Desert. On May 29, 2013, the Ozone Mapping Profiler Suite (OMPS) on the Suomi NPP satellite captured this view of an expansive dust event in Africa and southern Europe. Plumes of dust
stretched over Turkey, Greece, Albania, Montenegro, Serbia, and part of
Bosnia-Herzegovina. Aerosol concentrations in the air are represented
by a quantity known as the aerosol index, with the highest
concentrations in deep red and the lowest in light yellow. The heaviest
loads of dust appear to be emanating from near the Bodele Depression in Chad.Dust storms that reach Europe are fueled by cyclones with marked
seasonal cycles. The most intense dust events to affect the Eastern
Mediterranean region usually occur in spring and early summer; by the
end of summer, a low-pressure system often emerges over the Balearic
Islands and pushes plumes toward the Western Mediterranean.As seen above, not all of the dust kicked up in Saharan dust storms
moves north. About 60 percent flows west toward the Atlantic, often drifting all the way to South America. Previous research has shown that about half the mineral dust that fertilizes Amazon soil originates in the Bodele Depression.While dust can have severe impacts on air quality close to the
source, dust from the Sahara is usually soaring at about 3 kilometers (2
miles) above the surface by the time it reaches Europe, explained NASA
atmospheric scientist Colin Seftor.
“It’s up so high that people on the ground aren’t really affected,
though if they look up they may notice the sky looks a bit milky or
opaque.”However, what goes up must eventually come down. Gravity pulls dust
earthward, and rain and snow accelerates the process. Colored rains
occur when raindrops fall through dust plumes. Heavier rainstorms
usually wash dust away before it is readily apparent, but brief, light
showers can leave noticeable color across the landscape. Red rains only
occur if dust particles contain enough iron oxide.Dusty rains occurs throughout Europe, but they are most common along
the shores of the Mediterranean Sea. Red rains are the rarest, but they
still occur regularly. University of Arizona researchers have compiled a
database with details of more than 500 events that have occurred since 1900. A few weeks before OMPS acquired this data, the Balkan Chroniclereported red rain in Bosnia-Herzegovina.

NASA Earth Observatory
image by Jesse Allen, using Suomi NPP OMPS data provided courtesy of
Colin Seftor (SSAI). Suomi NPP is the result of a partnership between
NASA, NOAA and the Department of Defense. Caption by Adam Voiland.

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